This invention involves a control process of an automatic control function of a steering system for an automotive vehicle, as well as a steering system and an automotive vehicle comprising such a control process.
Generally, automotive vehicles comprise a steering wheel secured to a steering system column which is operated by the driver to turn the front wheels of this vehicle.
Usually, steering systems comprise a booster device that generates a portion of the energy required to turn the wheels, so as to reduce the effort that the driver must deliver, in particular at low speed and standstill maneuvers.
In addition, certain steering systems comprise a motorization unit, automatically controlled by a control computer, that may act on the steering column and control the turning of the front wheels to perform certain specific functions, such as for low speed City Park maneuvers, or safety maneuvers such as avoiding an obstacle, or a return to the normal traffic lane in case of detecting an exit or a risk of exiting this lane (“PFIL”, “LKS”, “LKAS”).
The motorization unit for the control function may include in particular an electric motor or hydraulic cylinders of the booster device, to generate the effort that involves the rotation of the steering column and the corresponding turning of the wheels. The control computer that receives the sensor information, such as from a steering wheel angle sensor, implements a control process of this motorization unit, to produce a position control of the steering column required by the control function.
A known control process measures the angle difference between the actual position of the steering column and a theoretic position to calculate through a PID or RST type regulation system, for instance, the torque set point that the motorization unit must deliver to arrive at the angular position of the steering column that it has foreseen.
However, the driver may want to take over control again by applying, among others, an opposite force on the steering wheel, if he wants for instance to steer the vehicle himself by imposing a direction different from the one that the automatic steering control function seeks to achieve.
In this case, when the effort applied on the steering is opposed to the torque generated by the motorization unit, the angular difference between the real position of the steering column and the position requested by the function increases, and the position servo-control tends to generate a greater torque to arrive even better in achieving the calculated position.
Then, if the driver suddenly releases the steering wheel, one obtains a recall torque of this steering wheel generated by the motorization unit that is even greater than the angular difference. The steering wheel starts at a high speed, within the dynamics range given by the servo control system and the motorization unit, which is unpleasant, and can be annoying or even dangerous.